leetcode--Next Permutation

Implement next permutation, which rearranges numbers into the lexicographically next greater permutation of numbers.

If such arrangement is not possible, it must rearrange it as the lowest possible order (ie, sorted in ascending order).

The replacement must be in-place, do not allocate extra memory.

Here are some examples. Inputs are in the left-hand column and its corresponding outputs are in the right-hand column.
1,2,3 → 1,3,2
3,2,1 → 1,2,3
1,1,5 → 1,5,1

public class Solution {
    /**
	 * The algorithm is standard. 
	 * @param num --int array which represents a permutation
	 * @author Averill Zheng
	 * @version 2014-06-12 --world cup opening ceremony
	 * @since JDK 1.7
	 */
	public void nextPermutation(int[] num) {
		int length = num.length;
		if(length > 0){	
			int i = length - 2;
			while(i > -1 && num[i] >= num[i + 1]){
				--i;
			}
			if(i == -1)
				Arrays.sort(num);
			else{
				int j = i + 1;
				while(j < length && num[j] > num[i]){
					++j;
				}
				//swap num[i] and num[j - 1];
				int temp = num[j - 1];
				num[j - 1] = num[i];
				num[i] = temp;
				++i;
				j = length - 1;
				while(i < j){
					temp = num[j];
					num[j] = num[i];
					num[i] = temp;
					++i;
					--j;
				}
			}	
		}
    }
}